The extraordinarily formidable difficulties presented by heavy black tar formation in processes designed to produce liquid hydrocarbons from coal by any pyrolysis process are indicated by the fact that these heavy tars have posed insuperable problems in over 60 y of major R&D projects all over the world, particularly in Germany, France, Great Britain, China, Japan, and the U.S.. The work in the U.S. is particularly well documented; major expenditures by the Bureau of Mines, the Office of Coal Research, D.O.E., and the big oil companies have failed to yield a commercial process. The only pilot plant sufficiently successful to continue in operation is the Encoal Plant operated by a subsidiary of the Ziegler Coal Co. in Gilbert, Wyo. with DOE funding. It operates on Powder River Basin sub-bituminous coal. The liquid product is tar of such a high viscosity (the pour point is about 90° F.) that it has been utilized only in boiler furnaces built to operate on No. 6 residual fuel oil. The main justification for operating the plant is that the char produced has been dewatered and its physical structure altered to reduce the moisture it can absorb so that its heat content per pound is much increased, and this reduces shipping costs.
The bulk of the literature on coal pyrolysis published in the past 50 years has been concerned with either small-scale bench tests with small quantities of coal—generally less than a gram—or pilot plants employing gas-fluidized beds with coal through-flows of a few tons per day. The experiments in the former were generally carried out under exquisitely controlled conditions, often with only a few particles of coal in a vacuum or an inert gas atmosphere; there is no mention of heavy black tar formation in these reports. However, in the literature on small pilot plants one finds that, whatever they might obtain in the way of a liquid product, it was generally a heavy viscous black tar. This was ordinarily attributed to polymerization of lighter hydrocarbons by a sort of catalytic action of a high concentration of activated carbon fines. These fines are inherently entrained with the hydrocarbon vapors from a gas-fluidized bed because the flow of sweep gas required to fluidize the bed of coal particles is many times the flow of vapor evolved from the coal so that a great deal of fine particles is swept up out of the fluidized bed and entrained in the sweep gas. While the larger particles can be removed in a cyclone separator, those in the micron size range are carried off with the hydrocarbon vapor to the condenser. This solid particle contamination of the hydrocarbon liquid amounts to about 10% by weight of the condensed vapor, and presents formidable filtering problems as well as liquid quality degradation caused by polymerization.